Many current technology NOx reducing catalysts for application in the exhaust of a lean burn engine--including diesel and certain spark igniting engines--require hydrocarbon (HC) species to effectively reduce the NOx concentration in the exhaust. The amount of HC required to reduce the NOx content of the exhaust varies with the selected converter and engine combination. Some engine applications will not have enough HC in the exhaust to remove the required amount of NOx. Accordingly, one aspect of the present invention relates to the injection of HC into the exhaust in order to reduce the concentration of NOx in the exhaust. Certain inventive techniques for adding HC into the exhaust are described in co-pending applications.
With the addition of HC into the exhaust of certain engine/catalyst packages, the NOx emissions are effectively reduced, but the levels of HC and CO in the exhaust are significantly increased over the non-catalyst levels. In another aspect of the present invention, the additional HC and CO are brought back to acceptable levels by the addition of an oxidation catalyst downstream of a deNOx catalyst. This arrangement has been found to effectively reduce the NOx emissions from the engine without increasing CO and HC emissions above acceptable levels.
Certain aspects of the present invention are particularly applicable to catalytic converters for relatively large diesel engines. Because ceramic catalyst substrates cannot readily be extruded in diameters greater than about 12 inches, catalytic converters for relatively large diesel engines must necessarily include a plurality of ceramic catalyst substrates arranged in parallel in order to adequately treat the exhaust without introducing unacceptable back pressures on the engine. In extreme cases, such as diesel engines for large generators, the catalytic converter can require fourteen or more 10 inch diameter catalyst substrates arranged in parallel within the exhaust passageway. These compound catalytic converters present new problems not previously encountered in relation to relatively small engines. Thus, in still another aspect of the present invention, certain inventive structures are described to overcome the special problems that arise in relation to relatively large diesel engines.